Comparison of common bean (Phaseolus vulgaris L.) genotypes for nitrogen fixation tolerance to soil drying

Common bean is a major source of protein for many people worldwide. However, the crop is often subjected to drought conditions and its advantage in undertaking symbiotic nitrogen fixation can be severely decreased. The primary objective of this study was to compare the resistance of nitrogen fixation of 12 selected genotypes to soil drying. Twelve common bean genotypes of diverse genetic background were compared. Plants were grown in pots and subjected to soil drying over about 2 weeks. Nitrogen fixation was measured daily using a flow-through acetylene reduction technique. The plants were exposed to acetylene for only a short time period allowing repeated measures. The acetylene reduction rate of plants on drying soil was normalized against the rates measured for well-watered plants. Substantial variability was identified among genotypes in the threshold soil water content at which nitrogen fixation was observed to decrease. Genotypes SER 16, SXB 412, NCB 226, and Calima were found to have the greatest delay in their decrease in nitrogen fixation rates based on soil water content. These four genotypes expressed substantial tolerance of nitrogen fixation to soil drying. These experiments also resulted in data on the threshold soil water contents at which transpiration rates decreased. A decrease in transpiration rates at high soil water contents is potentially advantageous since it allows soil water conservation for use as the severity of the drought increases. There was a general trend of those genotypes with sustained nitrogen fixation rates to low soil water contents also expressing decreased transpiration rates at high soil water contents. This study identified genetic variation among common bean genotypes in their response of nitrogen fixation and transpiration to soil drying. Five genotypes (SER 16, SXB 412, NCB 226, Calima, and SEA 5) expressed the desired traits for water-limited conditions, which might be exploited in breeding efforts.